Overview of the Endocrine System Flashcards
Maintaining Immediate Internal Homeostasis
Function of endocrine system: Keep stable in face of changes in internal and external environment (constantly changing)
Includes regulating metabolism
Ex. pH, blood pressure, sodium concentration, digestion etc.
Functions of the Endocrine System
“TNTC”- To Numerous To Count
- Maintain Immediate Internal Homeostasis (Breath to Breath)
- Manage Long Term, More Complex Issues
Manage Long Term, More Complex Issues
Function of Endocrine System
Initiates and/or regulates:
1. Growth/Maturation: bodily growth (birth to adult), sexual maturation
2. Reproduction (As an overall process): all are affected and controlled by hormones-> sexual activity, conception, gestation, delivery, lactation
Similarities between endocrine and nervous systems
- Employ cells that SECRETE something as a “communicator”
- Secrete that substance into a space where it reaches another cell (goes somewhere to have an effect)
- Require receptors on the target cell to exert effect (cell cannot respond without receptors)
- Share some of the same chemicals as a communicating substance (hormone, neurotransmitter)
Mechanistic Differences Between Endocrine and Nervous System
- Endocrine: Delayed effect, can reach target cell any distance away, duration of effect is delayed/prolonged (minutes to days)
- Nervous: Immediate effect, reaches immediately adjacent target cells, duration of effect is immediate (milliseconds-seconds)
Neuroendocrine Control
- Endocrine and nervous systems “cooperate” to achieve goals
- Systems closely interact to achieve communication and body homeostasis
- Neural input can cause hormone release and vice versa
Tropic Hormones
“Controlling”- control other ENDOCRINE tissues and ultimately affects production/secretion of another hormone
-Target cells of tropic cells are always endocrine cells
Ex. ACTH (adrenocorticotropic hormone)- targets adrenal cortex (endocrine) to release cortisol (hormone)
Ex. TSH (Thyroid Stimulating Hormone)- targets thyroid gland (endocrine gland) to release thyroid hormone
Trophic Hormones (Non-Tropic Hormones)
“Feeding”-promotes growth of tissues or organs
Targets NONENDOCRINE tissues and exerts effects other than hormone production/secretion
Exclusively/Solely Endocrine Organs
Thyroid Gland, Parathyroid Gland, Adrenal Gland, Pineal Gland
Organs with Both Endocrine and Non-Endocrine Function (Multiple Functions)
Kidneys, heart, gonads, pancreas, stomach, small intestine
One Endocrine Gland/ Multiple Hormones
One endocrine gland may make multiple hormones with widely differing functions
Ex. Anterior pituitary gland has 6 hormones which target: The body at large (the growth hormone), the thyroid gland, adrenal gland, mammary gland and the gonads
Hormone vs. Neurotransmitter
The same chemical may be both a hormone or a neurotransmitter
Ex. Norepinephrine:
Neurotransmitter at postganglionic sympathetic neurons
Hormone from the adrenal medulla
Hormone Secretion
The same hormone may be secreted by multiple endocrine glands
Ex. Somatostatin- produced in both the hypothalamus and beta cells of the pancreas
Hormones and Multiple Target Cells
The same hormone may have multiple target cells and induce very different effects at each
Ex. The same 9 Amino Acid Hormone causes both:
Contraction of vascular smooth muscle as vasopressin and assists with concentration of urine as antidiuretic hormone (ADH)
Overall, both work together to increase blood pressure
Same Hormones with Differing Effects
The same hormone can exert different effects because the particular effect exerted ultimately depends on what types of cells have receptors for the hormone in question (same types of receptor are present on different types of cells)
Process: Same hormone, multiple targets (Light switch)
- Receptors for the same hormone can be placed on different types of cells at different sites in the body (ex. vasopressin will affect smooth muscle and kidney b/c of same receptors)
- These different cells built to do specific tasks
- When cell is exposed to the hormone, the receptor binds the hormone
- Hormone receptor complex “turns on” the targeted cellular machinery
- Cell then does what it’s built to do (the hormone simply flipped the switch)
Single Target Cell, Multiple Hormones
A single target cell may respond to multiple hormones
Ex. Hepatic Cells respond to: Insulin (by converting glucose into glycogen), glucagon (by converting glycogen into glucose)
Ex. Ovaries respond to: FSH by growing follicles (trophic) and LH by rupturing follicles (ovulating)
Classic Definition of Hormone
“To set into motion”- Causes things to happen
-A messenger substance (chemical) produced by a specific organ that is released in small amounts into the BLOODSTREAM to a target located at a DISTANT site (organ)
Extended Definition of Hormone
Classic definition and includes extra substances:
- Produced by tissues or individual cells (not just organs)
- Produced by multiple cells, tissues, organs (not just one)
- NOT transported by blood (autocrine, paracrine, exocrine, neurotransmitters)
Autocrine Substances/Hormones
- Not transported by blood
- Acts upon cell of origin (acts on itself)
Paracrine Substances/Hormones
- Not transported by blood
- “Next to”
- Diffused through local tissue to act upon adjacent cells (secreted by cells into interstitial space)
Exocrine Substances/Hormones
- Not transported by the blood
- Secreted outside the body
- Released to exterior of body or into body cavity (pheromones, GI hormones)
Neurotransmitters
- Not transmitted by blood
- Produced by a neuron and acts upon another neuron OR upon a non-neuronal target cell
Common characteristics of all hormones
Circulate and EFFECTIVE at LOW plasma concentrations, effective at low concentrations due to amplification of their signals, specificity of action of blood-borne hormones effected receptor specificity, receptor modulation, hormone disposal
Amplification of signals
Characteristic of all hormones
Effective at low plasma concentrations due to amplification of their signals
Through various mechanisms, one hormone molecule induces a response that individually causes multiple effects
Receptor Specificity
Characteristic of all hormones
Specificity of action of blood-borne hormones effected receptor specificity
For hormones transported by the blood, all cells of the body are exposed to that hormone. However, since only the target cells have the correct receptors, only they are able to respond to hormone exposure
Receptor Modulation
Hormone action requires the presence of appropriate cellular receptors, thus altering number/nature of receptors allows regulation of hormone action (down regulates and weakens response)
Numbers of hormones not fixed- both down regulation and up regulation are possible to decrease or increase magnitude of a given hormone’s effects to maintain homeostasis
Can also change in type of receptor (ex. FSH->LH to allow ovulation to take place)
Hormone Disposal
Liver and Kidney are the primary organs involved with hormone excretion (hepatic and/or renal disease may present for or at least include endocrine signs)
Degradation and Inactivation
Degradation of Hormone
HORMONES THAT GET USED
follows hormone action
Takes place intracellularly in the target cell
degraded hormone is then excreted by the kidney
Inactivation of Hormone
HORMONES THAT DON’T GET USED
Mostly takes place in the liver
Inactivated hormones excreted into bile or returned to plasma for urinary excretion
Four Major Groups of Hormones
Prostanoid (prostanoid=prostaglandins), Amine, Steroid, Protein/Peptide
Prostanoid Hormone Synthesis
Made from arachidonic acid at many sites
No affiliation with specific organs
Prostanoid Hormone Production/Release
Produced in a wide variety of cells and organs throughout body
Released immediately after synthesis
Chemically instable and rapidly metabolized (won’t float around in interstitial space or blood very long)
Action is limited to immediately adjacent cells (paracrine)-local effect, diffuse into interstitial space to neighboring cell
Prostanoid Hormone Actions
- Sensitize pain fibers
- Generate fever
- Affect +/- contraction of smooth muscle (vascular, GI, uterine)
Amine Hormones Synthesis
Amino acid derived- 3 major categories
- Serotonin (tryptophan derived)
- Catecholamines (tyrosine derived)- epinephrine/norepinephrine (made by adrenal gland) and dopamine (made by dopaminergic neurons throughout body)
- Thyroid Hormones- T3, T4 (Tyrosine derived)-made by and stored in thyroid gland
Peptide Hormones
Most abundant type of hormone
Affect a tremendously broad range of bodily processes
Only difference between peptide hormones and protein hormones is size (peptides smaller)
Peptide/Protein Hormone Synthesis
Synthesized on RER
As proteins, are WATER soluble, and therefore can be stored in membrane-bound vesicles within the cell of origin
Precursors are larger than finished products (proteins are cleaved off to make hormone active)
Storage vs Secretory Vesicle
Storage: Sitting still, holding contents, make ahead of time and hold for later
Secretory: Storage vesicle on its way to release contents
Release/Secretion of Protein Hormones
Upon cue, secretory vesicles migrate to surface membrane of cell, fuse with it and exocytose the hormone into the extracellular fluid
From there, the hormone diffuses to the nearby target or into the systemic circulation
Plasma Transport of Protein Hormones
Since proteins are water soluble (hydrophilic), protein hormones are simply transported and dissolved within the plasma
Do not need transport proteins because water soluble so can move freely in plasma
Overview Mechanism of Action of Protein Hormones
To affect a cell, any hormone or its signal must get into cell, Proteins are water soluble and so cannot themselves enter cell, Their receptors are therefore on the cell surface membrane (externally), Require a second messenger to carry that message intracellularly (ex. cAMP)
Stepwise Mechanism of Action of Protein Hormones
- Protein hormone binds to receptor on cell surface
- Hormone-receptor unit triggers conversion of ATP to cAMP
- cAMP triggers activation of protein kinases in the cytosol
- Kinases cleave proenzymes to the active form of the enzyme
- Enzyme causes biological effect
Ultimately-the design of protein hormone action is to change pre-existing inactive proteins to an active form - Therefore the onset of protein hormones is RAPID
Sex and Adrenocortical Hormones
Steroid Hormones
Sex Hormones: estrogens, progesterone, and androgens (testosterone)
Adrenocortical Hormones: glucocorticoids (cortisone) and mineralcorticoids (aldosterone)
Synthesis of Steroid Hormones
Mostly cholesterol derived
Cells of adrenal gland have a little bit of the enzymes that control SEX HORMONE production as well as gluco- or mineralocorticoid production (adrenal gland produces small amounts of estrogen and testosterone)
Release of Steroid Hormones
Essentially, steroid hormones are released as they are produced
Cannot be stored as the hormones themselves- steroid hormones are lipid-soluble and freely diffuse out of the cell once produced
No external cue required for release
Not an instant effect like peptide hormones (because they can’t be stored)
Transport of Steroid Hormones
Lipophilic (can’t float free in plasma)- not water soluble
Must move through plasma bound to transport proteins
Specific Steroid Binding Proteins Types
Individual proteins for individual hormones (not many)- thyroxine, sex and corticosteroid
High affinity but low total capacity-carriers bind their hormone efficiently and well, since there are fewer of this type of carrier a smaller total percentage of steroid hormones are carried this way (ex. 15 binding proteins each carrying 1 bucket which can only carry 1 specific protein)
General Steroid Binding Proteins
Albumins
Albumins transport many substances (including hormones)
Normally an abundance of albumins in plasma
Low affinity (general) but high capacity (since there is so much albumin)
Ex. 100 albumins each have 4 buckets and each bucket can carry 25 proteins
Regulating effects of Binding Proteins
- Acts as a “storage pool”-only about 1% is typically unbound (loose in circulation), amount free in blood depends on number of receptor cells expressed
- Increases Plasma Half Life (increases the half life of the protein in the plasma)- decreases renal clearance because protein bound so too big to filter, only the free form can be degraded (plasma-mostly water, proteins don’t like water so not free in plasma)
- Helps regulate hormone action- only unbound form is free to exert its effects (only unbound form is active)
- Reduced numbers of binding proteins affect amount of free hormone, therefore primary hepatic disease can affect endocrine system (liver makes albumins- not enough binding protein available leads to endocrine disease)
Mechanism of Action of Steroid Hormones
- In order to affect function of the target cell, the hormone or its message must get into the cell
- Steroid hormones are lipid soluble so receptors are intracellular (free in cytosol or within the nuclear membrane)
- Hormone first diffuses free from its protein carrier and approaches cell surface
- Steroid hormone then diffuses through cell membrane into the cell
- Binds to the intracellular receptor
- The hormone-receptor complex enters the nucleus
- The hormone-receptor complex binds to DNA at appropriate sites
- Protein synthesis is initiate so Gene expression is altered (direct protein synthesis de novo of a new protein from DNA)
- Steroid hormone action is a slower onset than proteins (proteins just activate pre-existing pro-enzymes)
Productoin Control of Locally-Acting Hormones
Hormones NOT transported by blood
Ultimate effect usually very local (paracrine, autocrine)
Chemically unstable, so break down quickly and half-life very short
No need for “rigamarole”
Production Control of Blood-Borne (“classic”) Hormones
Controlled via feedback systems for communication at distant sites
Structures involved in Feedback Systems
- Hypothalamus- portion of the BRAIN, produces certain hormones
- Pituitary Gland (aka Hypophysis)- an extracellular gland, produces and releases certain hormones
- Target organ or tissue- endpoint, the final structure that will produce the ultimate somatic effect
Negative Feedback Systems
Goal is bodily homeostasis, the most common form of feedback systems
Secretion and/or action of hormone decreases secretion of that hormone (can either be the level or hormone detected or the end result of the action)
Feedback loop may or may not directly involve the hormone itself (secretion or action)
-If it does- the increased blood level of the hormone that is detected and thereby reduce production and/or secretion of the hormone (ex. plasma cortisol senses itself and inhibits release of more)
-If it doesn’t- feedback arises from some consequence of hormones action- when consequence reaches certain level then hormone stops (ex. Body detects sodium concentration so renin release from kidney diminishes-> body detects sodium concentration not renin)
Long Loop Negative Feedback
Starts far away from site of production of hormone
Feedback originates at the target gland, acts back upon either the hypothalamus or the pituitary gland
Short Loop Negative Feedback
Feedback originates at pituitary gland
Goes back to hypothalamus
Ultra Short Loop Negative Feedback
Feedback originates at hypothalamus
Hypothalamus acts on itself
Positive Feedback
Goal is NOT bodily homeostasis-> stimulate finite end result
Hormone secretion increases secretion of more hormone
Ex. insulin release continues until blood glucose returns to normal; oxytocin release in labor continues and increases until fetus is expelled
